A perc test costs money and a soil evaluation costs more. The USDA soil survey is free and tells you, before you spend either, whether the ground on this lot is the kind that usually passes or the kind that usually fails.
Real data, no fakes. FEMA NFHL · USFWS NWI · USDA SSURGO · USGS 3DEP & NHD · NOAA Atlas 14
SitePrior queries the USDA SSURGO soil survey for the parcel and returns the mapped soil series, its drainage class, its hydrologic soil group, and whether it is rated hydric. Those four fields are what a septic designer looks at first. The preview also shows slope and depth-to-stream, because a lot can have decent soil and still fail on setbacks or grade.
| Hydrologic group | What it means | Conventional septic outlook |
|---|---|---|
| A | Sand and gravel. Drains fast. | Usually passes. Watch for too-fast rates near wells or streams. |
| B | Loam and sandy loam. Moderate infiltration. | Best case for a conventional drain field. |
| C | Silty or clay loam. Slow infiltration. | Often marginal. Expect a larger field or an engineered system. |
| D | Clay, shallow bedrock, or high water table. | Conventional systems commonly fail. Plan on a mound, drip, or ATU system and a bigger budget. |
Drainage class sharpens the picture. "Well drained" and "moderately well drained" are what you want. "Somewhat poorly drained" or worse means a seasonal high water table, which most state septic rules treat as a limiting layer regardless of the perc rate. A hydric rating is a wetlands flag as much as a septic flag.
A perc test measures one hole on one day. The soil survey describes the whole map unit and its typical seasonal behavior. If the survey says Group D and somewhat poorly drained, a passing perc in August does not make the lot a good septic site. If the survey says Group B and well drained, a failed perc is worth a second look at test location and method. Either way, you walk into the site evaluation knowing what to expect and what the alternative system will cost.
It is accurate enough to decide whether the lot is worth a site evaluation. SSURGO map units are mapped at roughly 1:24,000 scale and can include small inclusions of other soils. Treat a good result as permission to spend on the perc test, and a bad result as a reason to negotiate or walk.
It is the USDA rating of how fast water infiltrates when the soil is thoroughly wet, from A (fastest) to D (slowest). Group C and D soils are where conventional drain fields most often fail and where alternative systems get specified.
The test fee itself is modest. The real cost is the alternative system: mound, drip, or aerobic treatment units commonly run two to four times the price of a conventional field, and some lots have no approvable location at all. That is the number the $29 report puts in front of you.
No. Septic permits require a site evaluation by the county or a licensed soil scientist in most states. SitePrior tells you what that evaluation is likely to find so you can decide whether to order it.